1. Field of the Invention
The present invention relates to a wireless communication device and a wireless communication method that perform a communication priority control in a wireless LAN (Local Area Network) communication system, particularly in a CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) system.
2. Description of the Related Art
In a CSMA/CA system, a terminal performs virtual carrier sensing for a certain random period before transmission and checks whether another terminal is communicating with a wireless base station. If another terminal is communicating with a wireless base station, the terminal waits for the completion of the communication and then performs actual packet transmission. All terminals have equal rights to transmission.
The virtual carrier sensing denotes an act of generating a random number within a predetermined CW (Contention Window) after a channel has become idled for an IFS (Inter Frame Space) period, determining a random period based on the random number, and performing a back-off control in the random period.
The back-off control denotes a control in which a calculated random value is set as an initial value, the value is reduced along with a lapse of time, and actual packet transmission is performed when the value becomes zero. The IFS is defined by the IEEE 802.11 wireless LAN standard and is a specific period in which an idle detection should be performed prior to the transmission.
The CW is a maximum value of the random value that can be adopted in the back-off and is a parameter required to realize user multiplexing. The IEEE 802.11 defines a minimum value CWmin and a maximum value CWmax of the CW. A random value is calculated using the value of CWmin in the back-off of the first transmission, and the back-off is performed after doubling the CW in every retransmission. The CWmax denotes an upper limit of the CW.
The back-off dependent on the randomness enables a plurality of terminals to share the same channel for communication. However, in the system, a plurality of terminals may simultaneously transmit packets. In that case, a packet collision occurs and the packets are not appropriately received, resulting in the degradation of the communication quality. Particularly, due to the above reasons, the transmission quality is significantly degraded in a real-time application such as the VoIP (Voice over Internet Protocol).
An example of a conventional technology related to the priority control in such a communication system includes the EDCA (Enhanced Distributed Channel Access) defined in IEEE 802.11e as shown in Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications: Amendment 7: Medium Access Control (MAC) Quality of Service (QoS) Enhancements. In the EDCA, any one of four types of priorities is applied to packets, respectively, and packet transmission rights are preferentially given to the packets with high priorities by shortening the transmission latency of the IFS, CWmin, CWmax, and so forth. This allows a relative priority control in a communication environment where various applications such as voice and data are mixed.
However, the conventional EDCA method only provides a relative transmission priority order of the packets with different priorities. The method is a technique that does not contribute to the quality assurance or granting of priority among terminals that transmit packets with the same priority. Therefore, the possibility of an occurrence of packet collision caused by a plurality of terminals simultaneously transmitting the packets with the same priority cannot be inhibited. When the packet collision occurs, there is a delay even if the retransmission succeeds, and a packet loss occurs if the retransmission fails. Furthermore, when a plurality of terminals perform the back-off to obtain the packet transmission rights, there is a transmission latency or a delay for obtaining the transmission rights for a terminal deprived of the retransmission rights by other terminals. The incidence of the disadvantages is higher as the number of the terminals increases. Particularly, the delay or the packet loss generated by the disadvantages leads to a significant degradation of quality in a real-time application represented by the VoIP.
An example of a method for solving such disadvantages includes a technology for controlling the packet collision by setting packet transmission right obtaining priority periods with different timings among wireless terminals for each VoIP packet generation cycle as shown in JP 2007-214795 A. Further, an example of a schedule setting technology of the priority period such as the one described in JP 2007-235445 A which includes JP 2007-214795 A. These technologies allow the terminals to perform distributed autonomous transmission scheduling, thereby realizing smooth VoIP communication.
However, the terminals have to always or periodically receive downlink packets for other terminals from an AP (Access Point) in the technology of JP 2007-235445 A. In other words, packets in relation to communication of other terminals have to be observed in a cell, such as VoIP, in which a plurality of wireless terminals that periodically transmit packets exist. Therefore, the intermittent reception is impossible during the packet observation, and battery consumption (power consumption) is large because the intermittent reception cannot be performed. Additionally, a large load is imposed on a terminal that continues to receive (continues to observe) downlink packets for other STAs (STAtion). In case of the overload, the packet transmission and reception process of the station may be affected by a delay or other reasons.